Computerized quantitative structure activity determinations over a broad range of chemical structures were investigated. A new heuristic method was designed to establish priorities and to select the optimal test system for antitumor screening. This method based on the statistical significance of molecular structure fragment (keys) was applied on a relatively small set of compounds in a variety of chemical classes. The results were statistically significant but considerable additional modification and testing on more and larger data sets is needed. The priority assignment must also satisfy the more stringent test of applicability in an operational environment. Experimentation is continuing to remove redundancy and increase the relevance of the structural fragments. Work on a large-scale model is in progress. Biological data in a machine-processible form are available in the Drug Research and Development screening program for hundreds of thousands of compounds in mouse tumor systems. Molecular structural keys that have been used to query the Drug Research and Development chemical information structure file are similarly available for these compounds. The results of this larger test are expected to be conclusive and, hopefully, useful.